This invention relates generally to the validity testing of documents and particularly to the optical scanning of paper currency transported through vending machine bill validators.
In order to prevent the acceptance of invalid bills there are devices which perform validity tests on the bills after they are received into a bill validator acceptor opening, and while they are carried to a bill stacking position by a bill transport system, such as that shown in commonly owned U.S. Pat. No. 5,310,173.
A commonly used test is performed by an optical scanner which compares the effect of light passed through the received bill with the known effect of light passed through a valid bill. If the test is satisfactory the bill is stacked and if not, in general, it is returned to the depositor.
An early system for checking validity of bills is disclosed in U.S. Pat. No. 2,941,187 in which light is passed through specific areas of a paper currency bill to be received by a light responsive sensor. Circuitry connected to the light responsive sensor receives signals responsive to the light intensity and produces a signal responsive to the difference of the light intensity. A control system is provided to receive the signal and become active when the signal is of a predetermined intensity so that the value and denomination are accurately determined.
Other systems, generally more complicated, are known. For example, U.S. Pat. No. 3,782,543 discloses a document recognition system which provides an optical analyzer for watermark, surface print, wire band and paper density which includes a light source and two photosensitive elements which scan both sides of a bill as it passes over a guide plate aperture.
U.S. Pat. No. 3,916,914 discloses the use of a primary light source in one location and a secondary light source in another location and utilizes a lower support plate and an upper pressure plate having openings fitted with abrasive resistant light transmissive discs. The bill to be tested passes between the plates. At the primary light source the light is directed through the disc, the note, a grid and an opening in the bill support plate to be received by a sensor connected to an analyzer having output signals indicating the light intensity. At the secondary infrared light source the light is directed through the disc and reflected back to be received by an infrared detecting sensor having output signals. The note must be accepted by primary and secondary members receiving the signals for acceptance of the note.
In other, less complicated, known devices the bills are transported between upper and lower transparent plastic or glass sheets having light sources below the lower sheet and light sensors above the upper sheet. This is a simple, effective and economic means of testing the bills but, unfortunately, the surfaces of the light transmissive plastic or glass sheets through which the bills are transported become worn and collect dust or dirt. Most of the dirt is carried by the bills and tends to abrade and build up on the transparent sheet surfaces which degrades the light transmissive performance by scattering and reducing the light reaching the sensors and adversely affects the reading of the light sensors.
The present invention solves these and other problems in a manner not revealed in the known prior art.